1. Field of the Invention
This invention relates generally to mooring systems for offshore vessels and Floating Production Units (xe2x80x9cFPUsxe2x80x9d) such as Floating Storage and Offloading vessels (xe2x80x9cFSOsxe2x80x9d), Floating Production Storage and Offloading vessels (xe2x80x9cFPSOsxe2x80x9d), Floating Storage Drilling Production and Drilling Units (xe2x80x9cFPDSOsxe2x80x9d) and in particular to turret mooring arrangements or systems where a turret is rotatably supported on the vessel, and where the turret is fixed to the sea bed by anchor legs so that the vessel can weathervane about the turret.
2. Description of the Prior Art
Single point mooring systems using bearings, bogies, sliding elements and hydrostatic bearings at the interface of the motion between the geostationary turret and weathervaning hull are commonplace. Typically, the turret is a rigid tubular structure which transfers not only axial (vertical) loads from the mooring legs and risers, suspended therefrom, but also moments induced by mooring leg loads, hull deflections and any concentric radial misalignment between the lower sliding bearings at the vessel bottom and the motion interface at or near the deck. The moments and loads exerted on the bearings, bogies, or sliding element system reduce the efficiency of the interface and require additional cost to accommodate loads imparted by the interface itself which does not contribute directly to station keeping or riser support. These moments, deflection, and misalignment problems are common to single point mooring turrets of all sizes, which have a rigid turret connected to a deflecting hull, and depending on the method used for their remediation, they can result in a reduction in efficiency of the motion interface of as much as fifty percent. As the diameter of the turret increases, the material remaining in the hull outside the moon pool decreases, further exacerbating hull deflection problems at the hull-turret interface.
A second problem is the lack of machining capability in diameters large enough to accommodate a large size turret necessary for running as many risers as possible from the sea floor to a FPSO. Although the traditional limit has been around 50 risers per turret, recent interest puts the number as high as 40 to 120 risers per turret with the risers arranged in two concentric rows around the circumference of the turret. Given minimum distances required between risers for riser installation, the avoidance of clashing among them, and the provision for capability of their replacement during service, the diameter of the turret at the riser interface begins to exceed the limit of current roller bearing technology and therefore requires bogie wheels at the motion interface. If bogie wheels (or simply xe2x80x9cbogiesxe2x80x9d) are connected to the rigid structure, at both the hull and turret side of the interface, any vertical deformation of the interface results in some bogies carrying more loads than their neighbors do. For full efficiency, the two surfaces must be described by a locus of points circumferentially, which is part of two perfectly parallel planes. In practice for very large turrets, such parallel plane condition is impractical to achieve especially during operation at sea.
Several methods have been proposed for eliminating or accommodating the moments, deflections, and alignment problems associated with mounting a rigid turret within a flexing hull.
Diameter Reduction
U.S. Pat. No. 5,517,937 shows a turret of large diameter at the riser connection below, narrowing to a small diameter at the bearing location above, through terminating the upper ends of groups of riser guide tubes at different heights.
U.S. Pat. No. 5,584,607 and European Patent Application 0 668 210 A1 show a configuration for riser connection where the chain table is increased in diameter below the lowest bearing and at an elevation which is below the hull. Because the diameter of the circumferential rows of risers is outside the diameter of the turret shaft, special risers pull in and connection hardware is shown.
UK Patent Application GB 2 297 530 A shows a framework attached to the bottom of the turret structure which splays the risers outward in a radial direction to achieve the clearance necessary for installation, clash avoidance and replacement. In this way, the turret shaft and its associated bearings and machining can be minimized, while offering a path for many risers.
Isolation of Bearing from Hull Deflection
U.S. Pat. No. 5,052,322 shows a turret bearing mounted on a vertical extension of the cylindrical moon pool, which is connected rigidly at its base at a height below the main deck and thereby closer to the neutral axis of the ship""s hull and at a distance away from that connection and through an independent rigid ring that isolates that bearing from its hogging and sagging deflection of the hull.
U.S. Pat. No. 5,266,061 inverts the concept shown in U.S. Pat. No. 5,052,322, by extending the moon pool downward and mounting the bearing below the waterline, to isolate the bearing from the deflections of the hull.
WO 98/31585 copies the cylindrical support and rigid upper ring concepts shown by U.S. Pat. No. 5,052,322; however, WO 98/31585 utilizes bogies in lieu of a roller bearing. It also allows the equalization of load among the bogies through mounting on elastic members capable of flexure in the vertical direction.
Moment Elimination
U.S. Pat. No. 5,782,197 eliminates the moment induced by misalignment between the lower and upper bearing through mounting the upper bearing on a rigid ring which is in turn mounted on elastic mounts which permit the turret to swing in a pendular fashion about a point which acts as the virtual center of the sphere described by the relative motion of the bearing about that point above its central axis. In this way the upper bearing avoids misalignment induced moments and is able to flex somewhat in response to mooring system induced moments.
Compliant Mounting Arrangements for Bearings
U.S. Pat. No. 5,860,382 shows a radial roller used to center the turret within the moon pool, accommodating the flexure of the hull and ovality of the turret through mounting that radial roller on a radially compliant spring.
U.S. Pat. No. 5,893,784 shows a bearing support structure compliantly mounted on a frame supported by springs in the vertical direction. These springs allow flexure of the hull while maintaining a fairly uniform circumferential loading around the bearing.
U.S. Pat. No. 5,957,076 shows hydrostatic bearings, which provide a uniform circumferential load between the hull moon pool and upper edge of the turret structure. They require precise machining, sealing and are still subject to deflection of the moon pool in the radial direction due to ovality.
Alternate Load Path
Both U.S. Pat. No. 5,913,279 and EP 0815 002 B1 show configurations which allow moments and deflections to be transferred into the upper bearing up to a point at which radial sliding bearings come into contact to limit further transmission of loads and moments.
Identification of Objects of the Invention
A primary object of the invention is to provide a turret support system which can accommodate a large number (such as from about 40 to 120) of risers.
Another object of the invention is to provide a turret support system which isolates the upper bearing from vertical hull deflections of hog and sag.
Another object of the invention is to provide a turret support system which isolates the upper bearing from radial hull deflections of the moon pool due to ovality.
Another object of the invention is to provide a turret support system which exerts vertical loads on the upper bearing without induced moments.
Another object of the invention is to provide a turret support system which increases tolerance to radial misalignment between the center of the lower bearing and upper bearing.
Another object of the invention is to provide a turret support system which increases tolerance to circumferential misalignment while maintaining a uniform sharing of load among all load-bearing elements.
The objects identified above along with other advantages and features are incorporated in a system which suspends a single point mooring turret from bogie wheels which are arranged to roll along a rail of the FPSO hull. The system utilizes a pendular suspension system including rocker beams carried by the bogies which help transfer a circumferentially uniform axial load without moment through the interface of motion between the vessel and turret, by decoupling system loads, inertial loads, and hull deflection induced loads across that interface. The bogies are seated on a rail which allows the bogies to roll about the circumference of the moon pool. Suspending the turret from the bogies decouples radial hull deflections due to ovality, because the rail is not connected rigidly to the turret in the radial direction. A set of bogie rocker beams is coupled to the bogies. The bogie rocker beams are coupled to a set of turret rocker beams which are coupled to the turret. In one embodiment the turret is suspended via chains, cables, rods, columns, or the like between the bogie rocker beams and the turret rocker beams. In another embodiment, the turret is directly connected to the turret rocker beams. The rocker beams have equal arm lengths, and thereby equally share the vertical load imparted to any one bogie, thus reducing any moment on the bogies. Moments induced by the mooring system or inertial moments are counteracted via radial rollers or sliding bearings at the deck edge and bottom edge of the turret/vessel interface. The radial rollers or sliding bearings also provide vertical stability and centering of the turret within the moon pool.